The present invention relates to catalytic reactors and processes for using the same. More particularly, the present invention relates to chemical reactors, and processes employing the same, having a catalyst bed and including a microporous membrane separating a reactant phase from a zone where a catalytic reaction occurs.
Strictly speaking, a trickle bed reactor is a chemical reactor with a catalyst bed wherein the fluid flow is in the trickle flow regime, that is, rather low fluid flow rates. A conventional trickle bed system contains a continuous gas phase with a dispersed liquid phase. Increasing the gas flow rate can lead to pulsed or spray flow regimes. On the other hand, for higher liquid flow rates the gas may become the dispersed phase, thereby resulting in bubble flow regimes. In general, then, trickle bed reactors require low flow regimes, i.e., Reynolds numbers for both the gas and liquid phases of less than about 1,000. Increased flow rates in trickle bed reactors can result in flow variations and instabilities. In addition, channelling, incomplete loading, or flooding can result in the locale of the catalyst bed.
In operation, a typical trickle bed reactor has a fixed catalyst bed positioned vertically. A liquid medium flows downwardly through the bed while a gaseous reactant stream flows countercurrently upward through the bed. For a reaction to occur, the gaseous reactant must diffuse into the liquid phase, then diffuse to the catalyst particles, and then react. The reaction products, if soluble in the liquid phase, are removed therefrom.
The total reaction mechanism in such a system thus includes the steps of diffusion and reaction. The reactants must diffuse into the liquid phase and then diffuse to the catalyst particles, and the reaction rate is thus affected by the rate of diffusion to the catalyst particles. Assuming a situation where the catalytic reaction occurs at the surface of the catalyst particles, the reaction rate is affected further by the reaction rate constant, the concentration of the reactants at the particle surface, and the surface area of the catalyst particles. The resulting reaction products must then diffuse away from the catalyst particles and back to the mainstream liquid flow. Accordingly, the final reaction rate, as controlled by the slowest of the aforementioned steps, is affected most by either the rate at which catalysis proceeds or the rate at which diffusion of the reactants and the products proceeds. The primary resistance to diffusion occurs at boundary layer areas, and thus it would be advantageous to increase both the gas and the liquid flow rates to decrease such boundary layers. It would also be advantageous to increase the flow of either or both fluid phases while avoiding channelling and other flow problems which would be expected to occur with a conventional trickle bed reactor.
There is a species of reactors which use hollow fibers, but these reactors are specifically directed to biological systems. See, e.g., U.S. Pats. Nos. 4,201,845 and 4,442,206. In these biological reactors, a cell culture is fixed to or isolated by a microporous membrane, while a nutrient supply flows adjacent to and on the other side of the membrane. One disadvantage of such systems is that the viability of the cells must be rigorously assured, especially regarding temperature, pH, oxygen concentration, and salinity and osmotic pressure. Also, the products of biological reactors must diffuse back across the membrane to be removed, and thus higher concentrations of product on the cell culture side of the membrane not only decrease yields but also threaten cell viability. Even further, biological reactors will yield by-products other than the specifically desired biologically derived product. The present invention suffers from none of the foregoing disadvantages.